With the latest update to Mac Safari browser, you can now add your favorite image as the background to the start page.
If you don’t have Safari set to open to the default start page, you can learn how to do that by clicking here.
Open Safari and look at the lower right.
click on the Settings icon.
Make sure you tick the box next to Background Image.
Move the slider bar at the bottom to select from one of the backgrounds Apple offers.
Click the + icon to choose you own image from your computer.
You know have a background image.